1. Field of the Invention
The present invention generally relates to titanium aluminide for precision casting, and more particularly to titanium aluminide which is not heat treated after a precision casting process but results in a cast with high creep strength.
2. Description of the Related Art
Titanium aluminide (TiAl alloy) possesses various advantages such as being lightweight, demonstrating satisfactory strength at elevated temperature and having decent rigidity. Therefore, the titanium aluminide is considered as a new favorable material for rotating parts of an aircraft engine and vehicle engine or the like, and there is an increasing tendency to put it to practical use.
Conventionally, as taught for example in Japanese Patent Application Laid-Open Publication No. 8-311585, Fe and/or V is added to TiAl alloy as a third element to improve castability and B is added to TiAl alloy to provide fine crystal grains. By adding these third elements, it has become possible to fabricate a complicated product by precision casting. It is also known from the above mentioned Japanese publication that TiAl alloy having improved room temperature ductility and/or processability is obtainable by optimizing heat treatment. TiAl alloy disclosed in this Japanese publication is referred to as the conventional TiAl alloy or titanium aluminide according to the prior art hereinafter.
However, studies of TiAl alloys are primarily focused on improvements of room temperature ductility so that developed TiAl alloys have relatively low creep strength. Particularly, satisfactory creep strength is not demonstrated beyond 700.degree. C.
In order to raise the creep strength of TiAl alloys, there is known a method of adding a third element (Mo, Cr, W, Nb, Ta, etc.) into a TiAl mother alloy. This method, however, considerably degrades the precision castability of TiAl alloy so that a product with a complicated shape cannot be fabricated.
To overcome the above problems, the inventor proposed a novel TiAl alloy and casting method using the same in a copending U.S. patent application Ser. No. 09/217,673, entitled "TITANIUM ALUMINIDE FOR PRECISION CASTING AND METHOD OF CASTING USING TITANIUM ALUMINIDE" filed Dec. 21, 1998. The entire disclosure thereof is incorporated herein by reference and this TiAl alloy is referred to as TiAl alloy or titanium aluminide of earlier invention. The inventor disclosed how to heat treat the TiAl alloy in order to have a desired (or controlled) structure. The creep characteristic and precision castability are both improved according to this teaching. In particular, the improved creep strength demonstrates a value ten times (or more) greater than the conventional TiAl alloy without deteriorating the precision castability.
However, this TiAl alloy includes a trace amount of .beta. phase precipitated in the structure in an as-cast condition. The .beta. phase has an adverse effect on the room temperature tensile strength so that a particular heat treatment is required to disperse the .beta. phase. This raises the manufacturing cost. If this TiAl alloy is used to fabricate rotating parts of an aircraft engine which are not generally manufactured on a mass production basis, the resulting products are satisfactory both in terms of mechanical property and cost, but if it is used as a material for rotating parts of an automobile engine which are manufactured on a mass production basis, the products have desired mechanical characteristics but entail a high manufacturing cost.